This invention is directed to a method of rejuvenating molecular sieve or catalyst containing molecular sieve, and a method of using the rejuvenated molecular sieve or catalyst to make an olefin product from methanol. In particular, the invention is directed to rejuvenating a silicoaluminophosphate (SAPO) molecular sieve or SAPO catalyst by freeze drying the molecular sieve or catalyst prior to converting the methanol feed.
Silicoaluminophosphates (SAPOs) have been used as adsorbents and catalysts. As catalysts, SAPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. In particular, the use of SAPOs in converting alcohols or ethers to olefin products, particularly ethylene and propylene, is becoming of greater interest for large scale, commercial production facilities.
As is known in the development of new large scale, commercial production facilities in the commodity chemical business, many problems arise in the scale up from laboratory and pilot plant operations. Scale up problems arise in catalytic reaction systems in which large scale operation will be several orders of magnitude larger than typical pilot scale facilities. For example, conventional laboratory scale processes of making olefin products from oxygenate feed are conducted with catalyst loads of about 0.1 to 5 grams. Conventional large pilot plant operations may utilize as much as 50 kg of catalyst, making on the order of 20 kg/hr ethylene and propylene product, but this is nevertheless minuscule in comparison to what a large scale, commercial production facility would produce, if one were in existence today. Large scale, commercial production facilities, can require a catalyst loading of anywhere from 1,000 kg to 700,000 kg, producing anywhere from 600 to 400,000 kg/hr of ethylene and propylene product.
Operating large scale, commercial production facilities clearly presents great challenges in the development of the catalyst production-to-use chain. The term xe2x80x9cproduction-to-use chainxe2x80x9d refers to the entire area of activities beginning with the production of molecular sieve, including such activities as receipt of starting materials, on through the crystallization process. Also included in the production-to-use chain are intermediate activities which include formulation of the sieve with binders and other materials, activation of the manufactured sieve and finished catalyst; storage, transport, loading, unloading of molecular sieve and finished catalyst; as well as other practices associated with the handling and preparation of the sieve and finished catalyst for its ultimate use. The production-to-use chain ends at the point when the molecular sieve is introduced into the reaction system. For purposes of this invention, the end of the production-to-use chain does not necessarily mean the instant when the molecular sieve is introduced into the reaction system, since large scale systems are very large and instantaneous measurements are not practically feasible. In large scale systems, the production-to-use chain may be considered as completed some time within 12 hours of loading catalyst into the reaction system.
Since information to date relating to production of olefin products by catalytic conversion of oxygenate feedstock has been limited to laboratory and small pilot plant activities, little if any attention has been paid to the problems associated with the intermediate activities in the production-to-use chain. For example, little attention has been focused on the impact of storage, transport, etc. on catalyst activity, since small scale activity is rather easily manageable. While today only relatively small quantities of catalyst are prepared, stored and transported, large quantities of materials will need to be handled for commercial operations. Commercial operations may be require that large quantities of molecular sieve catalysts be stored for considerable periods of time and at multiple locations.
As the management of sieve and catalyst in the catalyst production-to-use chain expands in volume and complexity, a likelihood exists that millions of dollars will be tied up in catalyst inventory, and the value of the sieve and catalyst will be lost if quality, as compared to that of freshly prepared and calcined catalyst, is not maintained. Loss of quality will necessarily translate to loss of product quality, as well as loss of product quantity.
Although some work has been published relating to the intermediate activities in the catalyst production-to-use chain, few of the problems associated therewith have been addressed. For example, U.S. Pat. No. 4,681,864 to Edwards et al. discuss the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability, and that stability can be improved by using a particular activation process. In this process, organic template is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400-800xc2x0 C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses a particular method of calcining SAPO catalyst containing alumina as a binder. The method calls for heating the catalyst to at least 425xc2x0 C. in the presence of an oxygen depleted gas, and then in the presence of an oxidizing gas. The object of the heating process is to maintain the acid activity of the catalyst.
Briend et al., J. Phys. Chem. 1995, 99, 8270-8276, teach that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data is presented, however, which suggests that over at least the short term, this crystallinity loss is reversible. Even over a period of perhaps two years, the data suggest that crystallinity loss is reversible when certain templates are used.
EP-A2-0 203 005 also discusses the use of SAPO-37 molecular sieve in a zeolite catalyst composite as a commercial cracking catalyst. According to the document, if the organic template is retained in the SAPO-37 molecular sieve until a catalyst composite containing zeolite and the SAPO-37 molecular sieve is activated during use, and if thereafter the catalyst is maintained under conditions wherein exposure to moisture is minimized, the crystalline structure of the SAPO-37 zeolite composite remains stable.
Researchers at ExxonMobil Chemical Company has recently discovered that activated SAPO molecular sieve will exhibit a loss of catalytic activity when exposed to a moisture-containing environment. This loss of activity can occur between the time the catalyst is activated and even after as little as one day of storage. Although ways have been found to inhibit loss of catalytic activity, it would be highly beneficial to find a way to reverse the decrease of catalytic activity in a molecular sieve exposed to a moisture-containing environment.
In order to overcome the various problems associated with decrease of activity of a molecular sieve due to contact by moisture, this invention provides a way to reverse such decrease, i.e., to rejuvenate the molecular sieve. In general, this invention provides a process for rejuvenating a molecular sieve which comprises providing molecular sieve having a methanol uptake index of less than 1; and freeze drying the molecular sieve until the methanol uptake index is increased by at least 10%.
Preferably, the methanol uptake index is increased by at least 50%, more preferably the methanol uptake index is increased by at least 100%, and most preferably the methanol uptake index is increased by at least 500%. It is also preferred that the molecular sieve be provided having a methanol uptake index of less than 0.5, more preferably a methanol uptake index of less than 0.3, and most preferably, a methanol uptake index of less than 0.15.
Desirably, the molecular sieve is selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, ALPO-5, ALPO-11, ALPO-18, ALPO-31, ALPO-34, ALPO-36, ALPO-37, ALPO46, the metal containing forms of each thereof, and mixtures thereof Preferably, the molecular sieve is selected from the group consisting of SAPO-17, SAPO-18, SAPO-34, SAPO-35, SAPO-44, SAPO-47, the metal containing forms of each thereof, and mixtures thereof.
In a preferred embodiment, freeze drying is carried out at a pressure of 0.001 mtorr to 700 torr. Preferably, freeze drying is also carried out at a temperature of xe2x88x92200xc2x0 C. to 0xc2x0 C.
The invention also provides a method for rejuvenating molecular sieve containing catalyst which comprises providing molecular sieve containing catalyst having a methanol uptake index of less than 1; and freeze drying the molecular sieve containing catalyst until the methanol uptake index is increased by at least 10%.
The invention is further to a method of making an olefin product from an oxygenate which comprises providing molecular sieve containing catalyst having a methanol uptake index of less than 1; freeze drying the molecular sieve containing catalyst until the methanol uptake index is increased by at least 10%; and contacting the freeze dried molecular sieve containing catalyst with an oxygenate to produce olefin product. In a preferred embodiment, the freeze dried molecular sieve containing catalyst is contacted with an oxygenate at a temperature of 200xc2x0 C. to 700xc2x0 C. Preferably, the olefin product comprises ethylene and propylene. Desirably, the ethylene and propylene are separated into at least two product streams. The separated ethylene and propylene can then be contacted with a polyolefin-forming catalyst to form polyethylene and polypropylene.
The invention will be better understood by reference to the Detailed Description of the Invention when taken together with the attached FIGURE and in association with the appended claims.